It has been known for some time that different regions of the brain have different oxygen tensions. It is also known that different macroscopic regions of the brain exhibit differing tolerances to hypoxia/ischemia. However, it is not known if the variation in oxygenation correlates with tolerance to hypoxia/ishemia. We are assessing regional brain pO2 using LiPc implanted at specific sites in anesthetized rats. The animal is then subjected to a graded hypoxia protocol. Normoxic and hypoxic pO2 values in the brain are compared. The regions being assessed are cortex, hippocampus and basal ganglia, regions which are known to have differing hypoxia tolerances.